If his parents were incarcerated, that might delay things until they were released. In some cases, the clock stops on parental-rights termination proceedings until the parent(s) are released.
If the plan was reunification he may have been staying with a foster-only family that didn't want to adopt, or legally couldn't. (You could foster, for example, if you're seventy years old with health issues, but might have difficulty adopting a younger child. Not in the child's best interest if the adoptive parents die while they're still minors, kwim?)
Once it became clear the parents couldn't/wouldn't get custody back, they'd move him to a foster/adopt home.
And then things don't work out for a variety of reasons. Reasons for moving a kid to another home might be:
-- Illness, divorce, or other issues that might make the prospective parents ineligible before divorce proceedings are completed. A DUI or domestic violence conviction might be a good one.
-- Allegations involving the child, or another child, in the home might cause the foster/adopt home to be closed. i.e., the parents have another troubled kid who, pissed off at the foster parents, claims to the social worker, "foster daddy touched me!" to get the foster father in trouble -- and every kid in the home gets removed and placed elsewhere.
-- Disagreements between children. Foster kids are quite often troubled kids. The kids are fighting, or one kid accuses anothe other of inappropriate touching, etc. And they split them up or remove one.
-- Kid not happy in the home. And the parents may be awesome parents, but if the kid doesn't like something about the home -- say, they're being (necessarily strict) -- the child can tell his social worker something (lie) to get moved. "Daddy hit me!" -- Kid gets moved, home gets closed. Really, kid didn't like the seven PM curfew.
-- The kid has health problems or a mental health diagnosis that would scare off prospective parents. Look up reactive attachment disorder, and fetal alcohol syndrome, both of which tend to scare off prospective parents. HIV+ would also make finding an adoptive home difficult.
-- Prospective parents who want to adopt piss off a social worker. Unfortunately, this happens more often than anyone likes to admit. Say you have an inept social worker -- the parents start rocking the boat wanting more services, or whatever, for the kid. Either to "punish" the parents or simply to no longer have to deal with them, the social worker moves the kid to a different home.
-- The kid is behaviorally difficult. If you're trying to make for a sympathetic protag, this might not work.
-- Social worker privately doesn't like interracial adoptions. Social worker cooks up another story for why she moved the cute little black kid from the nice white family to the nice black family. Not legal, but it's easy to find an excuse to move a kid under other pretexts and this DOES happen.
Those are all reasons kids might be moved from home to home.
One OTHER possibility is that the foster parents do not WANT to adopt, but are willing to remain foster parents for the child until they age out. Reasons for this might include:
-- Kid has mental health issues, or a high risk of developing mental health issues, and the parents don't want the liability that adopting the kid would cause. They want to keep the option of having the child removed. If you adopt the kid and then reverse the adoption because the kid is now a dangerously unstable teenager and a threat to everyone else in the house, you often have to pay child support to the state. If you adopt a kid and then they set the neighbor's house on fire; you're responsible. If it's a foster kid, the state's insurance pays.
-- Also, when you adopt a kid, state payments are often cut, and the child may or may not be eligible for medicaid. Since many kids in foster care have serious issues -- mental health, physical problems -- this is a real concern. Mental health, in particular, is not often covered by private health insurance. That ADHD RAD bipolar five year old who just killed a puppy and doesn't talk ... is going to need a lot of very expensive therapy. Unless the foster parents are millionaires, they may not be able to cover the health care privately. (Incidentally, look up elective mutism if you want something that would make for a sympathetic hero, and would also likely trick the foster parents into thinking the kid was more troubled -- or perhaps mentally handicapped -- than he really was.)
I also know someone who didn't adopt a foster kid until he was in his late teens. It was gaming the system a bit, but they waited until he was older because it made him eligible for a full ride state scholarship to college. Kids adopted who were younger weren't eligible.
-- Leva